Recently, it has been discovered that ceramic materials can be sintered in a microwave field at temperatures substantially below those needed in a conventional thermal field. Because of this recent development there is a significant need to be able to determine the sintering behavior of these materials and to monitor the dimensional changes during the microwave sintering process. However, to monitor and measure the dimensional changes during the microwave sintering process a dilatometer which is compatible to a microwave furnace is required.
Typically dilatometers have a variable temperature furnace in which the test specimen rests between a flat surface and a movable object, such as a ceramic push rod, that extends outside the furnace. Temperature induced changes in the length of the specimen are transmitted through the rod to a mechanical, optical or electrical system for amplifying and measuring that change. These instruments can be used to make precise measurements of changes in length resulting from small temperature changes or to plot variations in the rate of linear expansion or contraction over a broad temperature range.
Among the least sophisticated dilatometers in common use are those in which the push rod is coupled to a dial gauge and the dilation of a specimen is read directly from the gauge. Such dial gauge dilatometers are simple to use and inexpensive, but generally are suitable only for applications that do not demand great precision.
A far more precise type of dilatometer is one in which the dilation sensor is a linear variable differential transducer which translates specimen dilation into electrical signals that can readily be amplified and recorded. In such a sensor, the core floats freely in the coil and each of these elements is separately supported at its ends by a pair of compound cantilevered springs. These springs permit independent and frictionless axial movement of the suspended element, but restrain radial or transverse movement. This independent and frictionless axial mobility of the core and coil facilitates calibration of the sensor and renders it extremely sensitive to minute changes in specimen length, thereby making possible exceptionally accurate measurements of expansion or contraction. However, the dilatometers described above are not microwave compatible. Therefore, to monitor and measure dimensional changes of a ceramic sample during a sintering process in which the sample was heated by microwave energy, it was necessary to develop a microwave compatible dilatometer.